DECAGON Fundamentals of Water Activity

Improving the supply. FDA & USDA Compliance.

odern scientific methods are ■ Water activity is a critical required to enforce the Federal measurement in determining the Food, Drug, and Cosmetic Act. shelf-life and safety of and other M substances. Laws to ensure the wholesomeness of foods and the safety and efficacy of Pure, safe, and wholesome food. drugs would be impractical without reliable methods of laboratory he U.S. food, drug, and cosmetics analysis to determine whether Tlaws are intended to assure the products are up to standard. Food and consumer that foods are pure and drug scientists in both government wholesome, safe to eat, and produced and industry must know the normal under sanitary conditions; that drugs composition of products to distinguish and medical devices are safe and them from those that are defective. effective for their intended uses; that They investigate the toxicity of cosmetics are safe and made from ingredients, study the causes of food appropriate ingredients; and that all poisoning, and test the potency of methods is: labeling and packaging is truthful, vitamins and thousands of drugs. OFFICIAL METHODS OF informative, and not deceptive. Their investigations also cover the ANALYSIS OF THE adequacy of controls over processing, ASSOCIATION OF OFFICIAL packaging, and storage practices. Any ANALYTICAL CHEMISTS. action taken by the FDA must be This 1,094-page book of tested methods, published since 1895, is an based on scientific facts which can be Visit our website. supported in court. The principal internationally recognized guide to www.decagon.com/aqualab/ authority relied on for laboratory analytical procedures for law enforcement. 1 ■ When a substance is added to lower water activity, the result can be complicated.

Ideally, an inert material could be added which would decrease water activity without any other effects such as increased ionic strength and decreased surface tension.

For instance, can be added to one reaction mixture and to another.

The amounts can be controlled so that the resulting compounds have identical water activity.

Table 1. Common spoilage organisms and their aw limits for growth.

Microbial Group Example aw Products Affected Sa1monella species Normal bacteria Fresh meat, 0.91 Normal yeast Torulopsis species 0.88 Fruit juice concentrate Normal molds Aspergillus flavus 0.80 Jams, Jellies Halophilic bacteria Wallemia sebi 0.75

Xerophilic molds Aspergillus echinulatas 0.65 Flour

Osmophllic yeast Saccharomyces bisporus 0.60 Dried fruits

2 Water activity basics for safety and quality in food products.

ATER ACTIVITY’S Water activity concepts— and decreased surface tension. In USEFULNESS as a food over 40 years old. reality, the choice of substance can Wquality and safety Until recently, water activity as a have a profound effect. For instance, measurement was suggested when it physiochemical parameter was mainly salt could be added to one reaction became evident water content could discussed in two scientific disciplines: mixture and sugar to another. The not adequately account for microbial physical chemistry and food amounts can be controlled so that the growth fluctuations. The water microbiology. In the former, it resulting compounds have identical activity (aw) concept has served the measures the thermodynamic free water activity. But the results of the microbiologist and food technologist energy of water and in the latter it is reaction will differ because of the for decades and is the most commonly used to define the lower limits of differing influences of salt and sugar used criterion for safety and quality. growth of food spoiling on biological reactions. Salt and sugar Its usefulness cannot be denied. . Microbiologists turned form different additional hurdles Biological cells and tissues are not to water activity measurements upon (see page 4). homogeneous solutions; neither are discovering that microbial spoilage of food products. Food products derive food occurs at widely varying levels of Water activity and hurdle most of their perceived consumer water content. Scott†, in the 1950’s, technology. benefits (apart from nutritional value) applied the water activity concept to Water activity should be regarded as from their physical and chemical describe the water availability for an external parameter like pH or differences. Water is not uniformly microbial growth. temperature. Under certain conditions, distributed, nor can it be disregarded it will act synergistically with other exclusively as solvent. he definition of water environmental parameters. Under T activity. other conditions it will be the sole Protecting against food spoilage. Scott defined water activity (aw) as parameter determining the outcome of Since yeast, molds and bacteria the ratio of the water a certain process. ■ require a certain amount of available over a food (P) to that over pure This is a compilation of thought from water to support growth, designing a water (P0). several authors. We hope it helps you product with an aw below 0.6 provides achieve your product shelf-life goals. an effective control. Water activity is w = P/P0 a —DECAGON defined as the equilibrium relative (ERH) divided by 100. Some Thus, multiplication of the water References: common spoilage organisms and their activity by 100 gives the relative Board, R.G. 1992. Ecosystems: microbes: aw limits are listed in Table 1. The humidity of the atmosphere in food— ecological concepts in food simplest way to reduce water activity equilibrium with the food. microbiology, The Society for Applied is with a process which drives off Bacteriology: Symposium series #21, pgs. 29S & 140S, Blackwell Scientific water— cooking, baking or R.H. (%) = 100 x aw dehydration. The high-heat processes Publications, Boston. also use the lethal properties of heat, Water activity is a better index Beuchat, L.R. 1981. Microbial stability as while dehydration or freeze drying for microbial growth than water affected by water activity, Cereal Foods only work by lowering the aw to a content. World, vol. 26(7), pgs 345–349. level that curbs growth. Water activity better predicts the growth of microorganisms because Hagerdahl-Hahn, Barbel. 1986. Water Binding free water microorganisms can only use activity: a possible external regulator in and food design. ‘available’ water, which differs biotechnical processes. Enzyme Microb. The second method involves tying considerably depending on the solute. Technol., vol. 8, June 322–323. up the free water by the addition of On average, ions bind the most water, Kuntz, Lynn A. 1992. Keeping solutes, usually or sodium whereas polymers bind the least microorganisms in control, Food Product chloride. This creates an imbalance in water; sugars and peptides fall into an Design, Aug '92, pg. 44–51. osmotic pressure which draws the intermediate position. At the same water from cells. Food designers face molecular concentration, salt lowers Montville, Thomas J. 1987. Food new challenges in maintaining the water activity more than sugar. microbiology: concepts in physiology sufficiently low aw with many of and metabolism, pgs 2–11, CRC Press, today’s fat replacers. Fat, which does When a substance is added to lower Boca Raton, FL. not contribute to the free water, is water activity, the result can be † replaced by water or a gel to provide complicated. Ideally, an inert material Scott, W.J. 1957. Water relations of food spoilage lubricity. These gels do not reduce aw could be added which would decrease microorganisms. Adv. Food and additional control methods are water activity without any other Res., vol.7, pgs 83–127. necessary to prevent spoilage. effects such as increased ionic strength 3 improves . ■ Apart from the most important and commonly used hurdles such as temperature, pH, and water activity, there are many others of potential value. EVERAL YEARS AGO, HURDLE TECHNOLOGY* was developed as a Snew concept for the realization of safe, stable, nutritious, tasty, and economical foods. It employs the intelligent combination of different preservation factors or techniques to achieve multi-target, mild but reliable preservation effects.

Mild processing of foods with hurdle technology. Many promising hurdles have been identified so far, although application of the idea in the food industry has been largely restricted to the meat sector. Recent studies, however, emphasize a much wider potential application, e.g., in bakery products, fish, and dairy products. More specifically, the concept was introduced into mild processing of fruits and vegetables. The design of new hurdles such as gas packaging, bioconservation, bacterocins, ultra- high pressure treatment, and edible coatings aided this development.

Consumers want fresher food products. Consumers demand fresher and more natural products. This prompts food manufacturers to use milder preservation techniques and could be stimulating the current trend to hurdle technology. There is an urgent need for new or improved methods producing stable and safe foods. The concept of hurdle technology Hurdle Technology also called: addresses this need. Hurdles disturb homeostasis. combined processes, combined methods, Homeostasis is the constant *combination preservation, combination reservation factors are tendency of microorganisms to techniques, or barrier technology. P hurdles to inhibit maintain a stable and balanced microorganisms. (uniform) internal environment. Hurdle technology deliberately Preservative factors functioning as combines existing and new hurdles can disturb one or more of the preservation techniques to establish a homeostasis mechanisms, thereby series of preservative factors (hurdles) preventing microorganisms from that the microorganisms in question multiplying and causing them to are unable to overcome (jump over). remain inactive or even die. Food These hurdles may be temperature, preservation is achieved by disturbing water activity, acidity, redox potential, the homeostasis of microorganisms. preservatives, and others. A crucial The best way to do this is to phenomenon in hurdle technology is deliberately disturb several known as the homeostasis of homeostasis mechanisms microorganisms. simultaneously.

4 Excerpted from Food Technology—Dec 1994, pg. 28.

Multiple hurdles affect product quality least. This multi-targeted approach is the essence of hurdle technology. It is more effective than single targeting and allows hurdles of lower intensity, improving product quality. There is the further possibility that different hurdles in a food not only have an added effect on stability, but can act synergistically.

helf-life of fermented sausage: S an example. Using hurdle technology, salami- type fermented sausages are produced that are stable at ambient temperature for extended periods. A sequence of hurdles is important at different stages of the ripening process. The first hurdles used are the preservatives, salt and nitrite, which inhibit many of the bacteria present in the batter. Other bacteria multiply, use up oxygen and thereby cause a drop in redox potential, which inhibits aerobic organisms and favors the selection of lactic-acid bacteria. These bacteria then proliferate, causing product acidification and an increase of the pH hurdle. During the long ripening process of salami, the initial hurdles gradually become weaker: nitrite is depleted, the number of lactic-acid bacteria decreases, redox potential and pH increase. However, since water activity decreases with time it becomes the main hurdle.

An increasing list of hurdles. ■ Annual Code of Federal ■ Since yeast, molds and bacteria About 50 different hurdles have Regulations (CFR), Title 21 require a certain amount of available been identified in food preservation. subpart references to water water to support growth, designing a Apart from the most important and activity. product with an aw below 0.6 provides In the definitions, §110.3(n), it says: an effective control against any commonly used hurdles such as “Safe-moisture level is a level of microbial growth. temperature, pH, and water activity, moisture low enough to prevent the there are many others of potential growth of undesirable microorganisms in value. Other hurdles include: ultrahigh the finished product under the intended pressure, mano-thermo-sonication, conditions of manufacturing, storage, photodynamic inactivation, modified and distribution. atmosphere packaging of both non- respiring and respiring products, edible The maximum safe moisture level for a coatings, ethanol, maillard reaction food is based on its water activity (aw). products and bacteriocins. Examples of An aw will be considered safe for a food foods preserved by combined processes if adequate data are available that are fruit juices and heat-processed, demonstrate that the food at or below ■ cured meat products. the given aw will not support the growth of undesirable microorganisms. 5 Measurement of water activity for product quality. Advantages chilled-mirror dewpoint sensors.

Chemical/biochemical reactivity. are often the result of moisture Water activity influences not only migration in multicomponent microbial spoilage but also foods. Moisture will migrate from chemical and enzymatic reactivity. the region of high aw to the region Water may influence chemical of lower aw, but the rate of reactivity in different ways. It may migration depends on many act as a solvent, reactant or change factors. For example, moisture the mobility of the reactants by migrating from the higher aw affecting the viscosity of the dried fruit into the lower aw cereal system. Water activity influences causes the fruit to become hard non-enzymatic browning, lipid and dry while the cereal becomes oxidation, degradation of vitamins, soggy. enzymatic reactions, protein denaturation, starch gelatinization, ree water vs. bound water. and starch retrogradation (see F Water activity instruments Figure 1). measure the energy status (sometimes referred to as free, Physical Properties unbound or active water) of the In addition to predicting the rates HE IMPORTANCE OF THE water present in a sample. A of various chemical and CONCEPT OF WATER enzymatic reactions, water ACTIVITY cannot be T activity affects the overemphasized. Water activity is a textural properties of measure of the energy status of the foods. Foods with high aw water in a system. More have a texture that is importantly, the usefulness of described as moist, juicy, water activity in relation to tender and chewy. When microbial growth, chemical the water activity of these reactivity, and stability over water products is lowered, content has been shown. (see undesirable textural Figure 1) attributes such as Microbial Growth hardness, dryness, staleness, and toughness Water Activity is a critical factor Fig 1 Water Activity Diagram—describing the that determines the of are observed. Low aw affects on microbial growth and reaction rates— products. Water activity, not water foods normally have content, determines the lower limit texture attributes of available water for microbial described as crisp and crunchy, portion of the total water growth. While temperature, pH, and while at higher aw the texture content present in sample is several other factors can influence changes to soggy. Also, water strongly bound to specific sites on whether an organism will grow in activity affects the flow, caking the components in the sample. a product and the rate at which it and clumping properties of These sites may include the will grow, water activity is often powders and granules. hydroxl groups of polysaccharides, the most important factor. The the carbonyl and amino groups of lowest aw at which the vast Controlling moisture migration. proteins, and other polar sites. Water activity is an important majority of spoilage bacteria will Water is held by hydrogen, ion- parameter in controlling water grow is about 0.90. The aw for dipole, and other strong chemical migration of multicomponent molds and yeast growth is about bonds. Additionally, some water is products. Some foods contain 0.61 with the lower limit for less tightly bound, but is still not components at different water growth of mycotoxigenic molds at available (as in a solvent for activity levels, such as cream filled 0.78 aw. water-soluble components). Many snack cakes or cereals with dried preservation processes attempt to fruits. Undesirable textural changes eliminate spoilage by lowering the 6 Reference: Fontana, Anthony 1998. Water activity: why it is important for . Proceedings of the First NSF International Conference on Food Safety. pgs.177-185.

■ AquaLab water activity meter. availability of water to equilibrium and to control the microorganisms. Reducing the aw boundary layer conductance of the also minimizes other undesirable sensor. chemical changes occurring during storage. The processes used to Speed and accuracy reduce the aw include techniques The major advantages of the like concentration, dehydration, chilled mirror dewpoint method, humecants, freezing, and freeze which is a primary method drying. These techniques control approved by AOAC International, spoilage by making water are speed and accuracy. Chilled unavailable to microorganisms. mirror dewpoint is a primary Because water is present in varying approach to measurement of energy states, analytical methods relative humidity based on that attempt to measure total fundamental thermodynamic moisture in samples don’t always principles. Since the measurement agree. Water activity tells the real is based on temperature story. determination, chilled mirror instruments make accurate Chilled mirror dewpoint theory. (±0.003aw) measurements in less In a chilled mirror dewpoint than 5 minutes. For some system, water activity is measured applications, fast readings allow by equilibrating the liquid phase manufacturers to perform at-line water in the sample with the monitoring of a product’s water vapor phase water in the activity. Processing changes can headspace of a closed chamber and then be made during production. measuring the relative humidity of With AquaLab’s chilled mirror the headspace. In the AquaLab, a technology, temperature control is sample is placed in a sample cup unnecessary for most applications, which is sealed against a sensor but available if required. block. Inside the sensor block is a fan, a dewpoint sensor, a ater activity—accepted and temperature sensor, and an Wapproved. infrared thermometer. The Water activity is an important dewpoint sensor measures the property. It predicts stability with dewpoint temperature of the air, respect to microbial growth, rates and the infrared thermometer of deteriorative reaction, and pure, wholesome, and safe. In the measures the sample temperature. physical properties. The growing past, measuring water activity was From these measurements the recognition of measuring water a frustrating experience. New relative humidity of the headspace activity is illustrated by the U.S. instrument technologies, like is computed as the ratio of Food and Drug Administration’s AquaLab, have vastly improved dewpoint temperature saturation incorporation of the water activity speed, accuracy, and reliability of principle in defining safety vapor pressure to saturation vapor measurements. ■ pressure at the sample regulations. The purpose of the temperature. When the water regulations are to detail the activity of the sample and the specific requirements and practices relative humidity of the air are in to be followed by industry to equilibrium, the measurement of assure that products produced the headspace humidity gives the under sanitary conditions and are water activity of the sample. The purpose of the fan is to speed

7 Are you in compliance with HACCP? Identifying your products critical control points.

ow does hat are critical control H HACCP W points (CCPs)? work? A CCP is defined as any point or There are seven procedure in a specific food system principles that where loss of control could result in serve as the an unacceptable health risk. Each CCP foundation for a will have one or more control HACCP system. measures to assure that the identified hazards are prevented, eliminated or Conduct a reduced to acceptable levels. For 1 hazard microbiological hazards, for example, a analysis to identify target water activity must be potential hazards established to prevent hazardous that could occur in organisms from growing. the food production process. hat are critical limits? W A critical limit is a maximum Identify the and/or minimum value to which a 2 critical biological, chemical or physical control points parameter must be controlled at a CCP ■ What does HACCP mean? (CCPs, see next column) - those points in to prevent, eliminate or reduce to an HACCP stands for Hazard Analysis and Critical Control Points. It is a way for the process where the potential acceptable level the occurrence of a industry to control and prevent problems, hazards could occur and can be food safety hazard. Critical limits may and ensure safe food by controlling the prevented and/or controlled. be based on factors such as: production process from beginning to temperature, time, water activity, or end, rather than detecting problems at Establish the critical limits that pH. A critical limit is used to the end of the line. HACCP identifies 3must be met at each identified CCP. distinguish between safe and unsafe where hazards might occur in the food A critical limit is a criterion that must operating conditions at a CCP. Critical production process, and puts into place be met for each CCP. limits should not be confused with actions to prevent the hazards from operational limits, which are occurring. By controlling major food risks, such as microbiological, chemical, Establish CCP monitoring established for reasons other than ■ and physical contaminants, the industry 4requirements to ensure each CCP food safety. can better assure consumers that its stays within its limit. Monitoring may products are safe. require materials or devices to measure or otherwise evaluate the World Water S OF JANUARY 26, 1998 HACCP IS NOW process at CCPs. LAW FOR meat, poultry and Aseafood processors in the US. Establish corrective action to be This science-based system will taken when there is a deviation 5 Work improve food safety and reduce the identified by monitoring of a CCP. In incidence of case a problem occurs, corrective attributed to meat and poultry actions must be in place to ensure no products. The old food inspection public health hazard occurs. program was based on a “see, smell and touch” approach that relied more Establish effective record keeping on detection of potential hazards than 6procedures that document the Water Activity Meters prevention. Today, microbiological and HACCP system is working properly. chemical contamination are of greater Records should document CCP importance to food safety. monitoring, verification activities and deviation records. DECAGON Post Office 835 Establish procedures for verifying Pullman, Washington 99163 7that the HACCP system is working properly. Verification consists of 509-332-2756 methods, procedures and tests used to fax 509-332-5158 determine that the HACCP system is in Visit our website. [email protected] www.decagon.com/aqualab/ compliance with the HACCP plan. www.decagon.com/aqualab/ 8